1. FIELD OF THE INVENTION
The present invention relates to a process of producing polyolefin foams, more particularly, to a process of producing polyolefin foams of any desired shape having skins possessing excellent smoothness and luster.
2. DESCRIPTION OF THE PRIOR ART
It is known to produce polystyrene foam at a high foaming ratio by pre-expanding expandable polystyrene beads containing an organic foaming agent such as butane, pentane, dichlorodifluoroethane, etc., filling the cavity of a mold having perforations for vapor passage with the pre-expanded polystyrene beads thus formed, introducing steam into the mold to further expand the pre-expanded beads and at the same time to fuse the surface of the beads to weld them to each other, and then cool the expanded, welded beads.
However, in the case of producing foamed moldings of a polyolefin, such as low density polyethylene, high density polyethylene, ethylene-propylene copolymer, polypropylene, ethylene-acrylic acid copolymer, ethylene-vinyl acetate copolymer, etc., it is difficult to prepare pre-expanded beads of the polyolefin by impregnating the polyolefin with an organic solvent, and hence a process has been proposed in which a polyolefin having compounded therein a foaming agent which generates a gas upon decomposition (hereafter, such a foaming agent is called a "chemical foaming agent") and a cross-linking agent is extruded into a sheet at a temperature which activates the cross-linking agent without decomposing the foaming agent and, after cutting the sheet into the proper dimensions, the sheet is heated in a mold to a temperature which decomposes the foaming agent to form foamed moldings. Foamed articles having a complicated shape cannot be obtained by such a process, however.
The inventors of the present application previously discovered an improved process for producing polyolefin foams having any desired shape (British Pat. No. 1,277,161). By this process, a polyolefin foamed article is produced by placing small pieces of a polyolefin foam (each having a mean volume of less than 0.2 cc and containing cells of less than 0.5 mm in diameter) in a mold having perforations for vapor passage, the mold confining the small foam pieces but being permeable to vapor and liquid in an amount of 95 to 100% by volume of the volume of the mold in most cases or 100 to 150% by volume in the case of compressing the small pieces. Steam is then introduced in the mold to heat the small foam pieces to a temperature between the melting point of the polyolefin and a temperature 50.degree. C. higher than the melting point, whereby the small foam pieces are welded to each other. This process is quite useful, but is accompanied by the problem that undesirable burrs are formed on the surface of the foamed article obtained at the portions positioned at the perforations of the mold. Further, even if the burrs are removed from the surface of the foamed article, marks remain on the surface of the article where burrs were removed, which reduces the commercial value of the article.
The reason for burr formation is that polyolefins are crystalline resins, and the melt viscosity of a polyolefin at molding is quite low, which is quite different from non-crystalline resins such as polystyrene, and molten polyolefin enters the vapor passage perforations of the mold during molding.
To prevent the formation of burrs, a process has been proposed in which small foam pieces are placed in a mold which has no perforations for the passage of vapor and then steam for heating is introduced into the steam chamber to weld the small foam pieces to each other. However, in such a process heating is insufficient, which makes it difficult to provide excellent foamed products.